The present invention relates to an ion implantation power supply and more particularly to a gaseous discharge nitriding system which facilitates the rapid extinction of unwanted arcing and the prompt re-establishment of desired glow discharge conditions.
As a method of nitriding metal surfaces to improve surface hardness, ion implantation is attracting increased interest. As is understood, such ion implantation can be performed by generating a glow discharge around the parts to be treated in a suitable nitrogen containing atmosphere. In order to be commercially feasible, however, such ion implantation processes must be operated at relatively high power levels where arcing may easily develop. As is understood by those skilled in the art, arcing will occur when the parts being nitrided are not perfectly clean. The impurities or pollutants present on the surface of the part being nitrided may provide a preferential point of breakdown by providing a local surface of high emissivity.
Once initiated, arcing has heretofore been difficult to control since the negative resistance characteristic of the glow discharge process requires that current limiting rather than voltage control be provided by the supply circuit powering the discharge. Rather than causing an increase in current, incipient arcing therefore tends to draw a disproportionate share of the total discharge current into a localized arc, e.g. at the dirty spot on the part or parts being treated. If this arc persists for any length of time, that point may be pitted or burned damaging the part. Further, with prior art supply systems, the speed with which the supply current can be reduced to extinguish the arc is limited and the speed with which the desired glow discharge can again be brought up to power is also relatively slow. This slowness in electrical response may in fact require that the gas pressure within the discharge chamber be temporarily reduced to permit re-initiation of the discharge, since the necessary concentration of ions for sustaining the glow discharge will have been depleted in the meantime.
Among the several objects of the present invention may be noted the provision of a novel ion implantation nitriding system; the provision of such a system which quickly extinguishes incipient arcing; the provision of such a system which facilitates rapid re-establishment of a glow discharge following a distinguishing of arcing; the provision of such a system which provides for precise control of discharge current; the provision of such a system which efficiently provides high voltage and high power levels appropriate for commercial ion implantation nitriding processes; the provision of such a system which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.